Circuit arrangement for varying the inductance of coils



NGV 359 1949 w. w. Bor-:LENS ET AL 2,488,370

CIRCUIT ARRANGEMENT FR VARYING THE INDUCTANCE OF COILS Filed Nov. 22,194e Patented Nov. 15, 1949 CIRCUIT ARRANGEMENT FOR VARYING THEINDUCTANCE OF COILS Willem Wigger Boelens, Gerard Hepp, Jacob vanSlooten, and Bernardus W. van Ingen Schenau, Eindhoven, Netherlands,assignors to Hartford National Bank and Trust Company, Hartford,

Conn., as trustee Application November 22, 1946, Serial No. 711,686 Inthe Netherlands September 6, 1945 Section 1, Public Law 690, August 8,1946 Patent expires September 6, 1965 This invention relates to acircuit for varying the inductance of a coil having a ferromagneticcore, as a function of a control current flowing through a magnetlsingwinding, the magnetisation of at least part of the core being determinedby the control current.

Circuits of this kind are frequently utilised for modulating the phaseor the frequency of electrical oscillations, or for obtaining automatictuning correction of receivers of electro-magnetical oscillations andmore particularly for acting upon the amplitude, the phase or thefrequency of electrical oscillations.

It is known in such circuits to decouple the coil and the magnetisingwinding by division of the coil and/or of the magnetlsing winding,preferably by division of the coil, and connection in opposition(push-pull connection) of the parts.

Applicant has found that the use of the circuit above describedfrequently involves undesirable effects in practice, which preventssatisfactory operation. Thus, for example, the said circuit often doesnot permit of obtaining undistorted modulation of electricaloscillations. The invention is based on the recognition that theoccurrence of these undesirable effects is due to insufcient magneticdecoupling of the coil with respect to the magnetising winding.

According to the invention, further decoupling of the coil with respectto the magnetising winding is obtained by that either the magnetlsingwinding itself, or a winding magnetically coupled thereto isshort-circuted with respect to the range of frequencies of theelectrical oscilla-V tions occurring in the coil, and/or by that eitherthe coil itself, or a winding magnetically coupled thereto isshort-circuited with respect to the range of frequencies of the controlcurrent.

Under certain conditions the coil itself may serve as theshort-circuiting winding coupled to the magnetlsing winding. For thispurpose the coil is composed of at least two parts connected so that theinductance of the coil is produced by parallel connection of the twoparts, the latter furthermore consituting a winding which is coupled tothe magnetlsing winding and short-circuited in itself with respect tothe range of frequencies of the electrical oscillations occurring in thecoil.

Use is preferably made of a short-circuiting winding (coupled to themagnetising winding). the natural frequency of which is higher than thehighest frequency of the frequency range of the said electricaloscillations.

In order that the invention may be clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawing.

3 Claims. (Cl. 332-51) Fig. 1 shows diagrammatically a circuit adaptedfor acting upon the inductance of a coil I as a function of a controlcurrent flowing through a magnetising winding 2. For the sake ofclearness each of the windings is shown as only one winding. Inpractice, however, the windings usually consist of a plurality of turns.

The coil I is assumed to pertain to an oscillatory circuit producingoscillations modulated in phase or in frequency. The modulation of theoscillations produced in the oscillatory circuit is obtained by varyingthe inductance of the coil I in the rhythm of the control current.Consequently, in the case assumed a high-frequency current occurs in thecoil I and a low-frequency current occurs in the magnetlsing winding 2.It is emphatically pointed out, however, that the assumptions made arenot essential to the invention.

Both the coil I and the magnetlsing Winding 2 are arranged on the sameferromagnetic core. constituted by two closed rings 3 3 and H- ofso-called high-frequency iron. The magnetisation of the rings and,consequently, the inductance of the coil I are determined by the controlcurrent flowing in the magnetlsing winding 2. If no particularprecautions were taken, a. low-frequency voltage would be induced in thecoil I by the low-frequency control current and a highfrequency voltagewould be induced in the magnetising winding 2 by the high-frequencycurrent, thus preventing proper modulation. For this reason the coil Iis composed of two parts surrounding respectively the rings 3' and 4'and connected in opposition (in push-pull) in such manner that theresulting low-frequency voltage induced by the magnetising winding isapproximately zero. It is also possible for one or more turns to bewound alternately on one and on the other ring, the turns beingconnected in series in the manner shown in the figure.

In the practical example the magnetlsing winding consists of turnssurrounding the two rings at the same time, and hence of one coll. Thisform of construction affords particular advantages which will bediscussed later in this i description. However, it is also possible forthe winding to be composed of two or more parts which in the present.instance surround the rings in the same sense and are connected inseries. It may be observed that in the case of a magnetising windingconstituted by one coil, use may be made with advantage of athree-limbed ferromagnetic circuit, in which event the magnetlsingwinding is arranged on one of the limbs (corresponding to thecombination of the ring-sections 3 and 4) and the halves of thehigh-frequency coil mesmo I are arranged on the other limbsrespectively.

Applicant has found that the circuit set out in the foregoing does notalways permit of obtaining undistorted phase modulation or frequencymodulation. According to the invention, this distortion is attributableto insufficient decoupling of the coil and of the magnetising winding,which may be understood as follows. The said division of the coil I andthe push-pull connection of the two parts, regarded superflcially, wouldallow almost complete decoupling of the coil and of the magnetisingwinding, neglecting saturation of the ferromagnetic cores. However, suchdecoupling can only occur if the two rings l-l' and l-l' and the twoparts of the coil are and remain wholly uniform both as regarddimensions and magnetic properties. Furthermore, this must be the casefor any value of the inductance brought about by the control current.This condition, however, never fulfilled in practice by the circuitdescribed. If a little crack occurs or extends, for example, in one ofthe two rings, the balance of the push-pull connection is completelydisturbed. Even if for determined values ot the control current completedecoupling were obtained, this decoupling would, as a rule, be disturbedupon variation of the control current due to the non-linear relationbetween the control current and the resulting inductance. Suchdisturbance usually causes serious dimculties, since it is possible thatpart of the magnetising winding have a natural frequency equal to thefrequency of the high-frequency oscillations produced in lthe coil I. Inthis case even a small coupling between the magnetising winding 2 andthe coil I is sufficient to bring the said parts into resonance. Due tothese resonance currents the inductance in the rings l'4 diifers fromthe value which it would require at the prevailing value of the controlcurrent, from which ensue considerably deviations from the properreactance value of the coil I and serious distortion in the modulation.

According to the invention, the said coupling is reduced almostcompletely to zero by the provision of a winding magnetically coupled tothe magnetising winding and short-circuited with respect to thefrequency range of the high-frequency oscillations. Further, saturationof the cores will produce a second harmonic of the frequency of thecurrent in the coils. This second harmonic may b'e reduced by the use ofthe invention. In Figure l this winding is designated 5; it isshort-circuited through a network t (shown diagrammatically) whichpasses oscillations of the frequency range of the high-frequencyoscillations but which has a high impedance to oscillations of thefrequency range of the low-frequency control current.

'The short-circuiting winding 5 is magnetically coupled to themagnetising winding 2 but is not magnetically coupled to the coil I. Theshortcircuiting winding may be constituted by a single turn, preferablylarge; lt is, however, also possible for the winding to be composed of aplurality of turns. Provision is usually made for the shortcircuitingwinding 5 to have a natural frequency higher than the highest frequencyof the highfrequency oscillations. The network 8 may be constituted, forexample, by a properly chosen condenser.

In order to ensure as ilrm as possible a coupling between theshort-circuiting winding and the magnetising winding, it is desirable toprovide the winding 5 between the ferromagnetic core l--l and themagnetising winding I; it la also possible to provide the winding Ubetween the ferromagnetic core 3%-4' and the coil I. In either case acoupling between the coil l and the winding 2 resulting from leakagefields, if any, is avoided as much as possible.

In connection with the foregoing it may be remarked that it isadvantageous to realize the identical to that shown in Figure 2, exceptforJ magnetising winding 2 as one coil, since in this case thepossibility of parts oi' the winding being set into resonance for thehigh-frequency oscillations is smallest. If, on the one hand, themagnetising winding would be equipped with two separate coils, onearound each ring (and if the action of the short-circuiting winding 5 isdisregarded for a moment), rthen the natural resonance of the coilswill, as a rule, differ from each other and the high-frequency voltagesinduced in the said coils will not neutralize each other evenapproximately, in other words, the known decoupling by means of divisionand push-pull connection is then not so good as in the event of thewinding 2 being constituted by one coil. I

Figure 2 shows a circuit in which the coil I serves at the same time asthe short-circuiting winding. Te coil I is constituted by two parts Iand I", which surround the rings 3 and l respectively. The inductance ofthe coil I is produced by parallel connection of the parts I andv I".The parts I and I also constitute a winding coupled to the magnetisingwinding 2 and short-circuited in series in itself, i. e. theseriesconnection of I and I" implies a short-circuitlng winding for thehigh-frequency oscillations but no short-circuiting winding for thelow-frequency` oscillations owing to the fact that the supply leadsinclude two networks 8 and 8" having a high impedance to oscillations ofthe frequency range of the control current and a low impedance to"capacity. The circuit shown in Figure 2, in which the two parts of thecoil I are connected in parallel, vaffords the further advantage thatthe two parts may be included in the high-frequency portion of thecircuit (not shown in the figure) so as to obtain a symmetricalarrangement in which each of the parts has the same capacity' withrespect to earth. The series-connection shown in Figure 1 does notexhibit this advantage.

Under certain conditions it is possible to short-1 circuit themagnetising winding itself with re-` spect to the high-frequencyoscillations. This can only be eected, however, if the number of turnsof the magnetising winding is comparatively small and. consequently, thecontrol current has a winding will yet come into resonance with thehigh-frequency oscillations and the effect aimed at is not achieved. Theshort-circuiting opera# tion may be effected, for example, by means of acondenser provided between the terminals of tha winding 2.

In order to reduce the coupling between the coil and the magnetisingwinding it is possible, also according to the invention, to shortcircuit either the coil itself, or a winding magnetically coupledthereto with respect to the frequency range oi' the control current.This step may be taken separately, or in combination with the other stepset out above.

Figure 3 shows a circuit in which the two steps are used in combination.The circuit is wholly the presence of a winding 8, coupled to the coil Iand short-circuited through a net-work 'I. The latter has a lowimpedance for oscillations of the frequency range of the control currentbut a high impedance for oscillations of frequencies falling within therange of frequencies of the highfrequency oscillations. The network 1may be constituted, for example, by a coil of properly choseninductance.

The operation of the short-circuiting winding 8 will be explained withreference to the circuit shown in Figure 4, which largely corresponds tothat shown in Figure 1. In this circuit, however, the short-circuitingwinding 5 is superseded by the short-circuiting winding 8 firstly hasthe effect that the magnetic fluxes in the two rings are rendered equalto each other (if they should be unequal) and remain equal at anyarbitrary value of the control current. If, furthermore the rings are ofsame diameter. the magnetic inductance in the two rings is and remainsequal, independently of the value of the control current. This, however,applies to the low-frequency magnetic fluxes but does not apply to thehigh-frequency magnetic fluxes, since the winding 8 is notshortcircuited with respect to high-frequency oscillations. By suitablyproportioning the push-pull connected parts I and I of the coil(supported as the case may be, by the provision of a shortcircuitingWinding 5), the coil I also may be decoupled for high-frequency currentwith respect to the magnetising winding 2. This may now be effected in asimple manner since this decoupling no longer depends on thelow-frequency control current. The desired decoupling once beingobtained, it is not disturbed by variations on the control current(which, of course, continuously occur in modulating the high-frequencyoscillations). This is attributable to the fact that invariably the samemagnetlsation prevails in each of the rings, so that the non-linearityin the relationship between magnetisation and control current can nolonger disturb the balance of the push-pull connection.

On comparing the functions of the short-circuiting windings 5 and 8 wecan say that the short-circuiting winding 8 tends to counteract thecause of asymmetries in the push-pull connection and that theshort-circuiting winding 5 tends to neutralize the consequences ofexisting asymmetries. Consequently, the windings, may advantageouslysupplement each other in their functions.

In the circuit shown in Figure 1 it is possible, if desired, toshort-circuit the coil I itself for the frequency range of the controlcurrent oscillations through a suitably chosen impedance, for example ahigh inductance. This may in some cases be effected with advantage,since an inductance of the high-frequency oscillatory circuit, whichincludes the coil, is connected in parallel with the coil I.

If the control current only comprises oscillations of very lowfrequencies, the networks 6', B, 8" may often be replaced by a directshort-circuit. At these low frequencies the voltages induced in theshort-circuiting Winding are so small that the resistance and/or theinductance of the shortcircuiting winding are in most cases sufficientlyhigh to prevent considerable currents of the frequencies contained inthe control current from flowing in the said winding, due to which themagnetising winding would otherwise be shortcircuited for thesefrequencies. 1f desired, resist- 6 ance wire may be used for theshort-circuiting winding.

What we claim is:

1. A circuit arrangement to provide a variable inductance comprising, afirst inductive winding element having an oscillatory current flowingtherein of a given frequency determined in part by the inductance ofsaid first inductive winding element, a second inductive winding elementhaving an oscillatory current flowing therein of a frequency lower thanthe frequency of the current in the rst inductive element, ferromagneticmeans to couple said first inductive element and second inductiveelement, and a third inductive winding element coupled to saidferromagnetic means and connected in parallel and in phase oppositionwith said first inductive winding element.

2. A circuit arrangement to provide a variable inductance comprising, afirst inductive winding element having an oscillatory current flowingtherein of a given frequency determined in part lby the inductance ofsaid rst inductive winding element, a second inductive winding elementhaving an oscillatory current flowing therein of a frequency lower thanthe frequency of the current in the first inductive element,ferromagnetic means to couple said rst inductive element and secondinductive element, a third inductive winding element coupled to saidferromagnetic means and connected in parallel and in phase oppositionwith said first inductive winding element, a fourth winding elementcoupled by the said ferromagnetic means to the said ilrst inductivewinding element, and in impedance network connected electrically inseries with said fourth inductive winding element and of propermagnitude to short-circuit oscillatory current frequencies found in thesecond inductive winding element.

3. A circuit arrangement to provide a variable inductance comprising, afirst inductive winding element having an oscillatory current flowingtherein of a given frequency determined in part by the inductance ofsaid first inductive winding element, a second inductive windingelement, having an oscillatory current fiowing therein of a frequencylower than the frequency of the current in the 'first inductive element,ferromagnetic means to couple said irst inductive element and secondinductive element, and a third inductive winding element coupled to thesaid ferromagnetic means, said third winding element shortcircuiting oneof the said oscillatory current frequencies and having a naturalfrequency of oscillation higher than the first inductive windingvelement oscillatory current.

WILLEM WIGGER BOELENS.

GERARD HEPP.

JACOB VAN SLOOTEN.

BERNARDUS W. VAN INGEN SCHENAU.

REFERENCES CITED The following references are of record in the nie ofthis patent:

UNITED STATES PATENTS Number Name Date 1,462,038 Hartley July 17, 19231,706,139 Boyajian Mar. 19, 1929 1,849,845 Mayo Mar. 15, 1932 1,953,519Tritschler Apr. 3, 1934 1,955,317 Wentz L Apr. 17, 1934 2,284,408DEntremont May 26, 1942

